This invention relates to techniques for starting computer systems and more particularly to a method for downloading executable code to a computer which is not provided with conventional boot code autonomous from the central processing unit.
All computing devices based on the model described by von Neumann require that a stored program be made available in memory for execution. Typically, such computing devices have a pre-loaded program (a "boot" program) in the form of executable instructions that contains sufficient logic to permit more extensive and higher-level programs to be loaded from external devices into the memory associated with the CPU. In early (pre-1975) computers, this boot program could be loaded manually directly through front panel switches connecting directly to registers to "stuff" the appropriate data and control bits to be transferring into a volatile memory associated with the CPU. However, modern computers lack a front panel, so this boot program is typically loaded into a pre-programmed read-only memory (ROM) device and automatically run upon application of power to the CPU. An example is the BIOS (Basic I/O System) Read Only Memory of an IBM-type PC.
In embedded computing devices, in which a microprocessor, ROM, and RAM are combined to perform a dedicated function, there typically is no external storage device, such as a disk drive. In these systems, either the stored program is typically unchangeable, in which case it is combined with the boot function onto ROM. In these systems, updating the stored program requires opening the device and changing a memory device, not an operation that a consumer would typically perform.
Alternatively, if a method is provided for a consumer to download a new stored program into the device, that typically requires that a boot program be resident inside the device and protected from consumer intervention.
The recent development of Flash EEPROM (Electrically Erasable Programmable Read-Only Memory) has made it possible to provide this function (namely, consumer download of new stored programs, typically an updated version) economically. Flash memory devices come in at least two forms--bulk erase, and boot-block. Bulk erase flash devices must be erased in toto. In order for a device using a bulk erase flash memory device to permit the consumer to update its stored program, the device must first erase its entire stored program, and then re-program it. If the process is interrupted after erasure but prior to enough of the stored program to be programmed (typically an interval of many seconds), the device is rendered inoperable. Furthermore, because the boot program has been erased, the device cannot be reloaded. Boot block flash memory devices have been devised to address this problem. In these devices, a portion of the memory is separately erasable, permitting a boot program to be stored there.
Unfortunately, boot block flash memory devices are expensive. In addition, they require that the memory device be pre-programmed prior to fabrication, which adds manufacturing cost to the device.
Alternatively, a separate memory device is required, which also adds manufacturing cost. What is needed is a scheme to permit code to be downloaded to a bulk erase flash memory device without requiring that the device be pre-programmed.